This invention relates to a circuit arrangement for operating a discharge lamp, comprising
a DC-AC converter provided with a branch A' comprising at least one switching element for generating a current of alternating polarity by being alternately conducting and non-conducting at a frequency f, PA1 a load branch B' coupled to the branch A' and provided with lamp connection terminals and with inductive means, PA1 a drive circuit E' for rendering the switching element conducting and non-conducting at the frequency f, in which the drive circuit E' is provided with a branch D' which comprises a series circuit of further inductive means and capacitive means, and with a branch C', which comprises a variable impedance, the drive circuit E being coupled to the inductive means in the load branch B', the branch D' being coupled to the switching element in branch A', and the branch C' being coupled to the further inductive means in branch D'.
Such a circuit arrangement is known from the Netherlands Patent Application 8701314, which corresponds to U.S. Pat. No. 4,935,672 (June 19, 1990). In the circuit arrangement described therein, branch A' comprises two switching elements which are alternately conducting and non-conducting. Branch C' shunts the further inductive means of the drive circuit.
By adjustment of the variable impedance, it is possible to set the frequency f of the current of alternating polarity and thus the power consumed by a lamp connected to the lamp connection terminals. It was found, however, that a comparatively small range of the lamp power can be controlled if branch C' consists of a variable resistance, which has the advantage of being comparatively inexpensive. This is a drawback which is caused by the fact that a reduction of the power consumed by the lamp to below approximately 80% of the rated lamp power requires such a reduction of the resistance setting that the quantity of power dissipated in the resistance increases to such an extent that the drive circuit is no longer capable of rendering the switching elements of branch A' conducting. The result is that the lamp extinguishes.
A variable inductance or a variable capacitance may also be chosen to form the variable impedance. A disadvantage of these options is that both a variable inductance and a variable capacitance are comparatively expensive components.